Oxidation of AISI 304L austenitic stainless steel at 850°C in O2: Microstructure evolution after breakdown of the Cr-rich oxide scale

摘要

Oxidation of austenitic stainless steel such as AISI 304L at high temperature leads to the formation of a protective duplex oxide scale, composed of a Mn-Cr spinel oxide (external layer) and a dense and adherent chromia Cr_2O_3 (internal layer). At high temperature, in high oxygen partial pressure, even if a thin Cr_2O_3 scale forms in the early stage of oxidation, the concentration and diffusion of Cr, particularly for fcc-austenitic stainless steel, cannot feed the further growth of Cr_2O_3.Then, the nodular growth of iron oxides takes place which induces a sudden increase of the oxidation rate. The phenomenon is called "breakaway oxidation". This study investigates the microstructural evolution of the oxidation affected zone of AISI 304L at 850 °C in O_2 flow using a combination of composional/elemental (TEM, Raman spectroscopy) and structural (EBSD) mapping techniques. For the longest oxidation time (312 h), SEM EDS observations on cross sections show the formation of deep oxidation pits underlined by a continuous Cr-rich oxide layer. TEM EDS mapping indicates that the pits are composed of Cr-rich oxides and cavities embedded in a Ni-rich metallic matrix depleted in Fe and Cr. Raman spectral mapping points out that the composition of the underlying Cr-rich oxide layer, Cr_2O_3 or FeCr_2O_4, is linked to the propagation of the oxidation front. EBSD crystallographic orientation map of the underneath substrate evidences a geometrical pattern of this Cr-rich oxide layer in relation with the substrate grain boundaries. A description of microstructure evolution is proposed from these experimental results. In the underneath steel substrate, Cr diffusion is faster in the steel grain boundaries than in the bulk. After about 100 h at 850 °C, Cr supply at the centre of some subsurface grains becomes insufficient to support the continuously growing Cr_2O_3 scale. Chromia is then gradually enriched in Fe and eventually locally converted in less protective FeCr_2O_4 spinel oxide. It results in fast outward cationic diffusion of iron and inward anionic diffusion of oxygen leading to the formation of two-layered oxide nodules. Depending on the composition of the surrounding alloy, an internal oxidation zone, composed of a Ni-Fe metallic phase and Fe-Cr spinel oxide, appears deeper in the alloy. The internal oxidation zone thickens, spreads laterally and eventually meets a grain boundary where Cr is easily supplied. A continuous Cr_2O_3 dense layer is then formed decorating the grain boundaries of the alloys and bordering the oxidation affected zone. The oxidation front is temporarily interrupted until the gradual conversion of Cr_2O_3 in FeCr_2O_4. Then, the inwards oxidation front progresses until a new surface defect is encountered where Cr supply is enhanced. It gives rise to a banded microstructure with alternated Cr-rich oxide layer (FeCr_2O_4 or Cr_2O_3) and two-phase layer (Fe-Cr spinel oxide and Ni-Fe metallic phase).